1. Field of the Invention
The present invention relates to an optical wavelength multiplexing and demultiplexing device used in a wavelength division multiplexing (WDM) optical communication network.
2. Description of the Related Art
An optical communication network employing a wavelength division multiplexing (WDM) technology has been extensively researched and developed in recent years.
FIG. 8 shows a schematic diagram of an example of an optical communication system, i.e., WDM network system, in which a plurality of optical wavelength multiplexing and demultiplexing devices are connected in series. One or more pairs of optical fibers OPF are provided for upward and downward communication lines as transmission paths.
The network system includes a plurality of optical amplifying repeaters REP in order to compensate losses in the optical fibers OPF. An optical amplifying repeater has two or more optical amplifiers OAMP for downward and upward lines.
Terminal offices A to D transmit a plurality of wavelength division multiplexed optical signals, i.e., WDM signals, each of which has a different wavelength, to one of the optical fibers. The optical wavelength multiplexing and demultiplexing device 1 distributes the WDM signals to transmission paths per a wavelength to transmit them to reception terminal offices A to D, which select wavelengths of the WDM signals and receive signals corresponding to the selected wavelengths.
The optical wavelength multiplexing and demultiplexing device 1 employed in the WDM network as shown in FIG. 8 is formed by combining optical add-drop multiplexers (OADM) 2 having a basic structure shown in FIG. 9.
The optical add-drop multiplexer 2 demultiplexes optical signals having some wavelengths (.lambda.d1,.lambda.d2 . . . .lambda.n) selected from the WDM signals, in which a plurality of wavelengths .lambda.d1, .lambda.d2 . . . .lambda.n propagating in the transmission fiber of a main group are multiplexed, to branch to a transmission fiber of a branch group, which is, for example, transmission fiber 10, directed to the terminal office B. Then, the multiplexer 2 multiplexes the remaining optical signals with optical signals inputted from the transmission fiber of an insertion group, for example, a transmission fiber 11 transmitted from the terminal office B to the optical wavelength multiplexing and demultiplexing device 1, in order to output to the transmission fiber of the main group.
It is normal to select the same wavelength of the optical signal to be demultiplexed as that of the optical signal to be inserted. The optical add-drop multiplexer 2 having the above-described characteristic can be produced by employing a wavelength multiplexing and demultiplexing element, such as a dielectric multilayer filter, a WDM coupler, a fiber grating, an AWG or the like. Accordingly, wavelength multiplexing and demultiplexing elements having various structures have already been proposed, in the first literature, titled as "An Experiment on Optical Add-Drop Multiplexer Using Fiber Grating and It's Limiting Factor" described in pp. 747 of a preliminary documentation of The institute of electronics, information and communication engineers, 1996, and second literature, titled as "Transmission Characteristics of Arrayed Waveguide NXN Wavelength Multiplexer described" in Journal of Light-wave technology Vol. 13 No. 3 March, 1995
As described above, it is required to use at least one pair of optical fibers for upward and downward lines in an actual WDM optical communication system. Accordingly, the optical wavelength multiplexing and demultiplexing device 1 is formed with the use of, at least, two or more optical add-drop multiplexers 2 shown in FIG. 9.
FIG. 10 shows a supposed structural example of the optical wavelength multiplexing and demultiplexing device 1 employing the optical add-drop multiplexers 2. As shown in FIG. 10, the optical wavelength multiplexing and demultiplexing device 1 includes a pair of the optical add-drop multiplexers 2 for upward and downward directions. Then, each of the pair of the optical add-drop multiplexers 2 is connected to an optical fiber 11 for demultiplexing and optical fiber 10 for insertion and multiplexing.
In the WDM optical communication network employing the optical wavelength multiplexing and demultiplexing device 1 shown in FIG. 10, a level schedule of optical signals in a normal section is changed because of some losses of optical signals, which have passed through the normal section on breaking the transmission fiber in a certain section off.
FIG. 11 is a schematic diagram of an example of a simple WDM network to overcome the shortage of the supposed system. In the network of FIG. 11, sections D, E, and F exist between terminal offices A, B, and C and the optical wavelength multiplxing and demultiplexing device 1, respectively. Optical repeaters REP required for keeping a predetermined signal level are provided in each of sections D, E and F.
Further, it is assumed that the optical wavelength multiplexing and demultiplexing device 1 employs a structure as shown in FIG. 10. It is also assumed that each wavelength of signals shown in a following chart 1 is allocated to communicate with each terminal office.
(Chart 1)
______________________________________ TERMINAL OFFICE A =&gt; B 2 TERMINAL OFFICE B =&gt; A 2 TERMINAL OFFICE B =&gt; C 3 TERMINAL OFFICE C =&gt; B 3 TERMINAL OFFICE C =&gt; A 1 TERMINAL OFFICE A =&gt; C 1 ______________________________________
Accordingly, an optical signal having a wavelength shown in FIG. 12 is propagated on each transmission path. Further, in FIG. 12, a circle marked shows a signal optical fiber.
Here, a case where an optical fiber cable of the section D is broken off in the above-described system will be considered. In this case, communication paths between terminal offices A and B or terminal offices A and C are broken off. However, a communication path between terminal offices B and C is kept, because the sections E and F are normal.
Accordingly, even if the optical fiber cable is broken off in the section D, it is preferred to give no effect on communication between the terminal offices B and C in order to operate a network system. However, for example, two wavelengths of .lambda.1 and .lambda.3 are propagated into the transmission path of the section E on normal condition. On the contrary, if the cable is broken off in the section D, only one wavelength of .lambda.3 is propagated to the transmission path of the section E.
In this way, if a transmission path in a certain section is broken off, a number of signals, which pass a transmission path in other normal section, is decreased.
On the other hand, an output control system has been used in the conventional optical amplifying repeaters, especially for a submarine optical communication system in order to make average optical power constant. Therefore, if a number of signals, which pass through the transmission path in a normal section, is decreased because of breaking the cable in a certain section off, a signal level schedule for one wavelength in the normal section is also changed at the end. Influence of non-linear effect of an optical fiber becomes strong according to the change of the signal level schedule, there has been a case where quality of transmission was deteriorated.
To overcome the above-described problem, it has been proposed to provide a function of controlling a gain of an optical amplifying repeater to be kept constant, so that there is no change of the signal level schedule, even if the number of wavelengths is changed.
However, there has been a problem that a structure of optical amplifying repeater becomes complex in order to obtain the function of controlling the gain of the optical amplifying repeater to be kept.
It is a key point that a signal level schedule in a normal section has been changed because some of optical signals passing in a normal section are fallen down by breaking the cable off in the certain section.